SPOKANE, Wash. -- As firefighters continue to put out hot spots, investigators are already combing for clues that could put them to the cause of an early-morning five-alarm fire at a lumberyard in northwest Spokane early Wednesday morning.

The blaze began around 2 a.m. at the Castle Rock Industries warehouse in the 1400 block of N. Elm Street.

Between 60 and 80 firefighters responded to the 5-alarm blaze, which was spread out over an entire city block.

Early on, firefighters entered the warehouse to fight the fire from the inside, but the building started to collapse, forcing the firefighters to back out and take up a defensive stand around the fire.

"The fire advanced very rapidly and started going up into the structural members," Spokane Fire Chief Bobby Williams said. "They pulled out, the building flashed over ... fire everwhere, and it became a defensive fire."

Witnesses said they heard several explosions with flames spiraling 200 feet into the air. Others said they could see the fire three blocks away and feel the heat from a block away.

"I was woken up by the sound of explosions," Spokane resident Ondrey Dezarn said. "I sat up and smelled smoke and wondered what in the world was going on and looked out my backdoor and saw nothing but an orange sky."

"It sounded like barrels, like five-gallon drums, exploding," another witness said. "You'd hear one about every one or two minutes exploding. There were about a dozen explosions."

Firefighters said the explosions were most likely propane tanks used with the fork lifts.

One firefighter was injured when a pole that supported the building fell on his leg. The firefighter was taken to a nearby hospital for treatment for a foot injury.

The flames quickly spread across the facility, completely destroying the warehouse. Three or four other nearby businesses were damaged by the blaze and several homes and buildings in the area lost power during the fire.

Firefighters eventually got the fire under control. As dawn broke, the destruction could be seen more clearly.

"Everything is gone. Semi-trucks melted down, all the businesses around, all the small businessesare torched up as well," Darrell Gilchrist, the brother of the owner of Castle Rock Industries, said.

Gilchrist said that the company has insurance on the property and said that several deliveries were due Wednesday and the company would work to get back on its feet as soon as possible.

Investigators are now on the scene working to determine what caused the fire.